Mónica Quiroga

A palladium tetra-coordinated complex as catalyst in the selective hydrogenation of 1-heptyne

Abstract Supporting on γ-Al2O3 a palladium complex with chloride and tridecylamine as ligands, it is possible to obtain an heterogeneous catalyst which is more active and selective for the 1-heptyne hydrogenation to 1-heptene than the classic Lindlar catalyst. At the same operational conditions, the supported palladium complex is also more active and selective than the same complex unsupported. The hydrogen pressure and the operational temperature showed to play an important role in the catalytic behavior of the catalysts under study. As determined by FTIR and X-ray photoelectron spectroscopy (XPS), the active species is the complex itself, which is stable under the reaction conditions. The obtained XPS results show that the palladium complex, supported or not, is tetra-coordinated, suggesting that its formula is [PdCl2(NH2(CH2)12CH3)2]. The highest activity and selectivity of the palladium supported complex can be attributed, at least partially, to electronic and geometrical effects.

Activated-Carbon-Heterogenized [PdCl2(NH2(CH2)12CH3)2] for the Selective Hydrogenation of 1-Heptyne

The complex [PdCl2(NH2(CH2)12CH3)2] has been heterogenized on two different activated carbons and tested as a catalyst for the semihydrogenation of 1-heptyne. The results are compared with those previously reported with the γ-Al2O3-supported complex. An important effect of the support porosity has been found. The location of the complex in the narrow pores induces shape selectivity. The effect of the support in concentrating the substrate close to the active species has been observed. A very active and selective catalyst has been prepared using an essentially microporous carbon.

Low metal loading catalysts used for the selective hydrogenation of styrene

A series of Group VIII metal catalysts was obtained for the semi-hydrogenation of styrene. Catalysts were characterized by Hydrogen Chemisorption, TPR and XPS. Palladium, rhodium and platinum low metal loading prepared catalysts presented high activity and selectivity (ca. 98%) during the semi-hydrogenation of styrene, being palladium the most active catalyst. The ruthenium catalyst also presented high selectivity (ca. 98%), but the lowest activity. For the palladium catalyst, the influence of the precursor salt and of the reduction temperature on the activity and selectivity were studied. The following activity series was obtained: PdN-423 > PdCl-673 > PdCl-373> PtCl-673 > RhCl-673 >> RuCl-673. As determined by XPS, differences in activity could be attributed, at least in part, to electronic effects.

Influence of ni addition to a low-loaded palladium catalyst on the selective hydrogenation of 1-heptyne

Semi-hydrogenation of alkynes has industrial and academic relevance on a large scale. To increase the activity, selectivity and lifetime of monometallic catalysts, the development of bimetallic catalysts has been investigated. 1-Heptyne hydrogenation over low-loaded Pd and Ni monometallic and PdNi bimetallic catalysts was studied in liquid phase at mild conditions. XPS results suggest that nickel addition to Pd modifies the electronic state of palladium as nickel loading is increased. Low-loaded Pd catalysts showed the highest selectivities (> 95%). The most active prepared catalyst, PdNi(1%), was more selective than the Lindlar catalyst.

Synthesis of methyl tert-butyl ether on sulfur-promoted ZrO2

The synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene in a gas–solid heterogeneous system over sulfur-promoted zirconias calcined at different temperatures between 400 and 700°C was studied. The reaction temperature was varied in the range 90–110 °C, and methanol to isobutylene molar ratios were changed from 1.0 to 1.3. The catalyst acidity was a direct function of the sulfur load, in turn a consequence of the calcination temperature. Temperature programmed reduction experiments allowed us to observe that the sulfur reduction process is different according to the calcination temperature. The highest activity was observed on the sample calcined at 600 °C, which was comparable with that of the reference catalyst Amberlyst 15 under the same conditions. A direct relationship between catalyst acidity and conversion was not observed. The injection of water completely poisoned the catalyst activity in a reversible manner.

ALTERNATIVE CATALYSTS FOR MTBE PRODUCTION

MTBE synthesis has been studied over catalysts having different acidities: zeolites, fluorine-promoted SiO2−Al2O3 and sulfur-promoted ZrO2. Some catalysts perform similarly to acidic resins and a clear relationship between acidity and activity is not apparent.

Catalytic investigation of PdCl2(TDA)2 immobilized on hydrophobic graphite oxide in the hydrogenation of 1-pentyne and the Heck coupling reaction

A Pd(II) complex with chloride and tridecylamine ligands (PdCl2(TDA)2) was immobilized on graphite oxide modified with the cationic surfactant octadecyltrimethylammonium bromide (C18TABr). Samples with different Pd loadings were synthesized and tested for the liquid-phase hydrogenation of 1-pentyne and the Heck coupling reactions of styrene–bromobenzene and styrene–iodobenzene. For the hydrogenation of 1-pentyne, the catalytic performance was found to be affected by the pretreatment procedure. For the Heck coupling reactions, the PdCl2(TDA)2/GO samples proved to be highly efficient catalysts. The catalytic activity was found to depend on the Pd loading for both reactions.

Metal and Precursor Effect during 1-Heptyne Selective Hydrogenation Using an Activated Carbon as Support

Palladium, platinum, and ruthenium supported on activated carbon were used as catalysts for the selective hydrogenation of 1-heptyne, a terminal alkyne. All catalysts were characterized by temperature programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. TPR and XPS suggest that the metal in all catalysts is reduced after the pretreatment with H2 at 673 K. The TPR trace of the PdNRX catalyst shows that the support surface groups are greatly modified as a consequence of the use of HNO3 during the catalyst preparation. During the hydrogenation of 1-heptyne, both palladium catalysts were more active and selective than the platinum and ruthenium catalysts. The activity order of the catalysts is as follows: PdClRX > PdNRX > PtClRX ≫ RuClRX. This superior performance of PdClRX was attributed in part to the total occupancy of the d electronic levels of the Pd metal that is supposed to promote the rupture of the H2 bond during the hydrogenation reaction. The activity differences between PdClRX and PdNRX catalysts could be attributed to a better accessibility of the substrate to the active sites, as a consequence of steric and electronic effects of the superficial support groups. The order for the selectivity to 1-heptene is as follows: PdClRX = PdNRX > RuClRX > PtClRX, and it can be mainly attributed to thermodynamic effects.

Sulfur Resistance of Pt-W Catalysts

The sulfur resistance of low-loaded monometallic Pt catalysts and bimetallic Pt-W catalysts during the partial selective hydrogenation of styrene, a model compound of Pygas streams, was studied. The effect of metal impregnation sequence on the activity and selectivity was also evaluated. Catalysts were characterized by ICP, TPR, XRD, and XPS techniques. Catalytic tests with sulfur-free and sulfur-doped feeds were performed. All catalysts showed high selectivities (>98%) to ethylbenzene. Activity differences between the catalysts were mainly attributed to electronic effects due to the presence of different electron-rich species of Pt0 and electron-deficient species of . Pt0 promotes the cleavage of H2 while the adsorption of styrene. The catalyst successively impregnated with W and Pt (WPt/Al) was more active and sulfur resistant than the catalyst prepared with an inverse impregnation order (PtW/Al). The higher poison resistance of WPt/Al was attributed to both steric and electronic effects.

NANOPARTICLES OF TUNGSTEN AS LOW-COST MONOMETALLIC CATALYST FOR SELECTIVE HYDROGENATION OF 3-HEXYNE

to be active and stereoselective for the production of (Z)-3-hexene, had the following order: 7.1WN/A > 8.5 WN/A ≥ 4.5 WN/A. Additionally, the performance of the synthesized xWN/A catalysts exhibited high sensitivity to temperature variation. In all cases, the maximum 3-hexyne total conversion and selectivity was achieved at 323 K. The performance of the catalysts was considered to be a consequence of two phenomena: a) the electronic effects, related to the high charge of W (+6), causing an intensive dipole moment in the hydrogen molecule (van der Waals forces) and leading to heterolytic bond rupture; the H+ and H- species generated approach a 3-hexyne adsorbate molecule and cause heterolytic rupture of the C≡C bond into C- = C+; and b) steric effects related to the high concentration of WO3 on 8.5WN/A that block the Al2O3 support. Catalyst deactivation was detected, starting at about 50 min of reaction time. Electrodeficient W 6+ species are responsible for the formation of green oil at the surface level, blocking pores and active sites of the catalyst, particularly at low reaction temperatures (293 and 303 K). The resulting best catalyst, 7.1WN/A, has low fabrication cost and high selectivity for (Z)-3-hexene (94%) at 323 K. This selectivity is comparable to that of the classical and more expensive industrial Lindlar catalyst (5 wt% Pd). The alumina supported tungsten catalysts are low-cost potential replacements for the Lindlar industrial catalyst. These catalysts could also be used for preparing bimetallic W-Pd catalysts for selective hydrogenation of terminal and non-terminal alkynes.